WASHINGTON, Feb. 23- More people may have been infected by the virus that causes bird flu than previously thought, and the number of deaths from H5N1 infection may also have been " overestimated," according to a study to be published Friday in the journal Science.
The study notes that the World Health Organization (WHO) has documented fewer than 600 cases of H5N1 influenza A infection -- also called avian infection -- in humans. However, the WHO's stringent criteria for confirming bird flu in humans -- often based on whether the symptoms are severe enough to result in hospitalization or even death -- doesn't account for the majority of infections, according to Taia Wang of the Mount Sinai School of Medicine in New York City and colleagues.
The researchers suggest that the current fatality rate of 50 to 90 percent, which is based on WHO criteria, is probably overestimated.
To reach such conclusions, Wang and colleagues performed a meta- analysis of 20 previous studies that assessed the blood serum of over 14,000 participants around the world. They estimate that somewhere between one and two percent of all those study participants actually had evidence in their blood serum of a prior H5N1 virus infection. However, people who have a limited amount of H5N1 antibodies in their blood serum often report no history of influenza-like symptoms. And in light of their findings, they suggest that the WHO's criteria for H5N1 infection do not account for the majority of infections; only the select few hospitalized cases that are more likely to be severe and result in poor outcomes.
The researchers said that further study, using a large-scale, standardized approach, is needed to determine the true rate of infection and deaths from the virus.
Since 2003, the H5N1 strain has infected 573 people and killed 336, notably in Indonesia, Egypt, China, Cambodia and Indonesia, according to WHO figures from 2003-2011.
The WHO says the primary risk factor for human infection with the avian virus appears to be direct or indirect exposure to infected live or dead animals, or contaminated environments. But experts fear the bird flu virus could mutate into a form easily transmissible between humans, with the potential to kill millions in a pandemic.
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